UKPassats

Hi everyone. Looking to get an idea of how to replace a rear near side seatbelt on a B7 passat saloon. I have bought a spare part as the existing one was partially chewed by my dog and needs replacing. My mate has had a look, as I'm useless when it comes to cars but he says he can't see how you can get past the metal housing around the parcel shelf and point of installation for the retractor assembly . It seems completely housed.

Apologies for my ignorance in advance, looking for any guides or advice.

Many thanks.